The present invention relates in general to the analysis of the condition of the lungs, and in particular to measuring the gas pressure in lungs.
Intrathoracic pressure during breathing is an important physiological parameter. It is used clinically to determine the effort that is generated by a patient during breathing. This effort is necessary to overcome the resistance of the airways while ambient air is moved into the respiratory zone of the lung (the alveoli) and back out. Intrathoracic pressures may be excessive during asthma attack when the airways are abnormally narrow, or during sleep when the upper airways are completely collapsed (i.e., sleep apnea) or narrowed and fluttering (i.e., snoring). Changes in gas pressure are known to be caused by various conditions in the lungs, such as increased airway resistance as caused by asthma, chronic obstructive pulmonary disease (COPD) and other conditions.
It is also important to know the intrathoracic pressure during artificial ventilation of critically ill patients. Excessive intrathoracic pressures are a common cause of pneumothorax, a potentially lethal condition in which air leaks from the lungs into the pleural space.
At any time a ratio between a pressure difference (xcex94P) between the airway opening to respiratory zone and a related flow (FL) is used as an estimate of airway resistance R=xcex94P/FL.
Several existing methods of lung pressure measurements exist. These include the body plethysmograph in which the body is enclosed in a pressurized chamber, the esophageal balloon, in which a catheter with a long and narrow balloon is inserted via the mouth or nose into the patient""s esophagus, airway opening pressure measurements, which requires intubation and which is used extensively for ventilated patients.
The invasive character and the limited accuracy of most of these methods makes them unusable outside a health care environment. For example, lung pressure measure measurements cannot be made at a patient""s home, nor can they be made without inconveniencing the patient such that it may be difficult for him to sleep.
One object of some preferred embodiments of his invention is to allow for non invasive measurements of gas pressure in the lungs. Preferably, gas pressure in lungs is measured indirectly, by measuring the velocity of sound propagating in and/or through the lungs.
One aspect of some preferred embodiments of the present invention relates to using breath sounds, especially tracheal breath sounds (snore or other vocalizations) for measurements of sound velocity in the lungs. Preferably, the sound velocity in and/or through lungs is measured by determining the difference in the time at which sound, generated at the onset of and/or during a respiratory event, reaches at least two distinct points on the surface of a person""s chest.
Another aspect of some preferred embodiments of the present invention relates to using a sound wave externally injected into a person""s chest instead of using sounds generated at the onset and/or during a respiratory event.
In preferred embodiments of the invention, the velocity is estimated from a difference in the phase of the sound at the two points. Changes in the phase indicate a change in velocity. Preferably, the phase utilized is that in a low frequency band, characteristic of sound transfer via the lungs. This is based on an observation that high frequency waves are preferentially transferred via the solid tissue and low frequency waves are preferentially transferred via the air in the lungs.
In preferred embodiments of the invention, a frequency domain transfer function is derived for sounds between the two points and rate of change of phase with frequency (slope) of the phase portion of the transfer function is used for the measurement or indication of velocity or velocity change.
In one preferred embodiment of the invention, a baseline value is determined at the start of a session (for normal breathing) and changes are used as an indication of changes in the condition of the lungs. Alternatively or additionally, the baseline values are determined in a particular session and measurements are periodically made to determine the patient""s condition over a period of time of days, weeks or years. Additionally or alternatively, the velocity measurements are compared to standard measurements which are indicative of different conditions.
In preferred embodiments of the invention, the measurements are made during segments of time which may extend from 50 msec to very long time intervals, even indefinitely.
In some preferred embodiments of the invention, only changes in the velocity (or phase) are considered important. Alternatively or additionally, absolute values of velocity are determined.
In preferred embodiments of the invention, the velocity/phase measurement is calibrated by measuring the velocity (phase) during a portion of the breath cycle when there is no air flow (at this point the alveolar pressure is equal to the external pressure) and also while the patient blows into a closed tube (no flow) at which time the pressure in the tube is the same as the alveolar pressure
There is thus provided, in accordance with a preferred embodiment of the invention, a method of determining the air pressure in a lung, comprising:
making a determination of the velocity of sound in the lung; and
estimating the pressure within the lung based on the determined velocity.
Preferably, the method includes determining the velocity and estimating the pressure separately for each lung.
In a preferred embodiment of the invention, making a determination comprises:
determining a difference in phase for sound between two positions with respect to the lung.
There is further provided, in accordance with a preferred embodiment of the invention, a method of determining air pressure in the lung, comprising:
determining a difference in phase for sound between two positions with respect to the lung; and
estimating the pressure within the lung based on the determined phase.
Preferably, the method includes determining a difference in phase separately and estimating the pressure separately for each lung.
Preferably, the phase is determined at a single frequency at which sound travels preferentially within the gas in the lung.
Preferably, the difference in phase is determined over a range of frequencies and including determining the rate of change of phase with frequency.
Preferably, the range of frequencies is characteristic of sound that travels preferentially in the air in the lung.
Preferably, the pressure is estimated based on the determined rate of change.
In a preferred embodiment of the invention, the sound is a multi-frequency sound signal. Preferably, the sound is injected into the body of a person being tested.
In a preferred embodiment of the invention, the sound is a swept sound signal.
Alternatively, the sound is a sound generated by a person being tested, for example a snore.
There is further provided, in accordance with a preferred embodiment of the invention apparatus for measuring air pressure in at least one lung comprising:
a) a first sensor operative for measuring a first phase of a sound at a first locality in a body when placed at said first locality;
b) a second sensor operative for measuring a second phase of a sound at a second locality in a body when placed at said second locality;
c) calculation circuitry that estimates the pressure based on a difference in the measured first and second phases.
Preferably,
a) said first sensor is adapted to be placed near the upper portion of a lung; and
b) said second sensor is adapted to be placed near a lower portion of the lung.
In a preferred embodiment of the invention, the apparatus includes a sound generator adapted to be placed outboard of said first sensor, such that the first and second sensors determine said phase of a signal traveling between the generator and the second sensor.
In a preferred embodiment of the invention, the apparatus comprises a third sensor adapted to measure a third phase of sound when placed at a third locality with respect to a lung.
Preferably, said second locality is adjacent to one lung and said third locality is adjacent to a second lung.